Magnetic induction flow meters utilize Faraday's law of induction to determine the flow rate of a through-flowing fluid. A magnetic field is generated perpendicular to the direction of flow. In this magnetic field, charges conveyed with the fluid generate, perpendicular to the magnetic field and to the flow-through direction, a voltage which can be picked off using electrodes. The measuring voltage thus identified is proportional to a flow rate based on the cross section of the flow channel. Flow meters of this kind have to maintain largely constant geometric dimensions in order to guarantee measuring accuracy irrespective of the through-flowing fluid pressure prevailing at any time. This pressure resistance is frequently achieved by means of a steel measuring tube through which the fluid flows. However, this measuring tube must not interfere with the electric and magnetic fields which permeate the fluid in the region of a measurement section. For this reason the measuring tube is provided with linings or inserts which are typically made from ceramics or from plastics materials. These meet the requirements of being electrically non-conductive and having virtually no effect on the magnetic field. At the same time they protect the metal wall of the measuring tube against corrosion. Moreover, inserts made from a plastics material have the advantage of being particularly easy to handle.
EP 0 895 066 A1 discloses a tubular insert for a magnetic induction flow meter. This insert is made from an elastomer, preferably synthetic rubber, which acts as the electrically insulating material. To ensure that the measurement section of the tubular insert is less flexible, steel bands are embedded in the rubber in this region. No reinforcement is present in the region of the two end sections. This document gives no indication as to how the steel bands are embedded in the rubber, that is to say, how a reinforced, tubular insert can be produced. The disadvantage of the known insert is that the rubber is not permanently secured to the inner surface of the steel bands and so the geometric dimensions of the tubular insert can vary with fluctuations in pressure.
US patent specification U.S. Pat. No. 5,773,723 discloses a magnetic induction flow meter having a measuring tube, the inner surface of which is lined with perfluoroalkoxy (PFA). This lining is stabilized in a measurement section by means of an embedded metal grid so that the inside diameter of the tube remains constant. The substantially tubular metal grid is formed by twisting a grid strip into a spiral and by welding together the mutually adjoining edges of the grid strip. Welded on the outside of the grid is a plurality of wires which extend in parallel to the tube axis and which are distributed uniformly around the circumference of the tube. The purpose of the wires is to ensure that there is a constant spacing between the inner wall of the measuring tube and the outer surface of the grid. The grid is inserted into the measuring tube and centered there by means of the welded-on wires. At the two end faces the tubular metal grid is welded to the inner surface of the measuring tube. The PFA lining material is inserted between an injection mold that has been inserted into the measuring tube and the inner wall of the measuring tube in an injection molding process, during which the PFA flows through the holes in the metal grid to fill the space between the grid and the inner wall of the measuring tube and to form a layer of constant thickness. One disadvantage of the known lining of a measuring tube is, however, the fact that it is relatively complex to produce. A further disadvantage is the difficulty of molding-on conically extending tube ends when the tubular metal grid is produced from a spirally twisted metal grid strip. Conically extending tube ends of the inside flow channel are, however, frequently provided on magnetic induction flow meters to enable a higher flow rate, and thus better measuring accuracy, to be achieved in the measurement section.